According to Whitehouse, Comey assured them he would confirm if an investigation exists "and the scope of their Russia/Trump investigation because he had not been able to at that point say that there was one."

Whitehouse and Graham both asked the Justice Department last week to provide any evidence that would support Trump's claims that former President Barack Obama wiretapped Trump Tower. The two are leading one of three Congressional investigations into Russia's interference in the US elections.

The FBI has not responded to a CNN request for comment on whether Comey will respond by Wednesday.

Graham expressed frustration Tuesday that the Trump administration has not yet provided any evidence of wiretapping and said he plans go "tough" on the administration during Wednesday's hearing.

The White House has vacillated on Trump's claims in the last two days. Press Secretary Sean Spicer said Monday that Trump did not mean literally that Obama had him wiretapped. But on Tuesday, Spicer said Trump was "extremely confident" he would be vindicated by the evidence.

"He feels very confident that what will ultimately come of this will vindicate him," Spicer said.

Democrats from the Senate Intelligence Committee -- whose investigation is separate from the one Graham and Whitehouse are a part of with the Subcommittee on Crime and Terrorism of the Senate Judiciary Committee -- said Tuesday they doubt Trump has evidence he was wiretapped, but the issue could easily be settled.

"It can all be put to bed with one phone call," said Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, before heading into a hearing of the group Tuesday afternoon. "I have seen absolutely no evidence of President Trump's claim about President Obama."

Warner said he expects the first public hearing from the Senate Intelligence Committee on Russia later this month. The Virginia Democrat said he wanted to hear from Roger Stone, a former Trump adviser whose contact with "Guccifer 2.0"-- the online persona who claims responsibility for hacking the Democratic National Committee -- also had Sen. John McCain wanting to question him.

"If we are investigating Russian interference, here is someone who has acknowledged that he has been in contact with the Russians," Warner said of Stone.

Stone has described his contact with Guccifer as limited to a "brief exchange with him on Twitter" and any suggestion otherwise, he told CNN, is "a fabrication."

The House Intelligence Committee scheduled its first hearing for Monday, and has invited a slate of current and former intelligence chiefs to testify.

Senate Intelligence Committee Chairman Richard Burr said Monday that he has asked the "appropriate people" for information about Trump's wiretap claims and said their answers have been "sufficient."

But Senate Democrats on the Intelligence Committee said they have not seen any evidence yet.

"I've heard nothing. But I'm strongly of the opinion there was no wiretapping," said Sen. Dianne Feinstein, a veteran member of the panel.

Warner also said he was not satisfied with the response to requests for information from the administration, saying, "I'm not sure why this is taking any time."